This invention relates to a method and apparatus for transverse cutting of multi-ply web material and, more particularly, to a continuous motion saw. A continuous motion saw is designed to cut a product in motion. Illustrative products are "logs" of bathroom tissue and kitchen toweling. The invention, however, is not limited to such products but can be used to advantage on other multi-ply products, such as bolts or continuous superposed plies of facial tissue, interfolded or otherwise.
The illustrative products, for example, are produced at high speed on machines termed "reminders". These machines start with a parent roll perhaps 10 feet long (2.3 meters) and about 8 feet (1.9 meters) in diameter--resulting from the output of a paper-making machine. The parent roll is unwound to provide a web which can be transversely perforated and then rewound into retail size rolls of at least 3.5 inches (89 mm) in diameter for bathroom tissue and kitchen toweling, viz. a cross-sectional area of 9.6 in.sup.2 (6190 mm.sup.2). Conventional high speed automatic rewinders can produce upwards of 30 logs per minute. These logs then are delivered to a log saw where they are moved axially for severing into retail size lengths. In the case of facial tissue, the product may be continuous (as in co-owned U.S. Pat. No. 4,052,048) and this also requires transverse cutting at regular intervals. In this case the minimum cross-sectional area is at least 3.5 in.sup.2 (2200 mm.sup.2).
Saws for making bathroom tissue, kitchen toweling, etc. started being used in the middle 1950s principally for toilet tissue and have come to be known as "log saws". Prior to that time, the retail size rolls were made by slitting the web on the rewinder. For example, spaced slitting wheels are shown operating against a slitting roll in co-owned U.S. Pat. No. 2,769,600. This proved unsatisfactory. When the wide web being processed had holes stemming from manufacture on the paper machine, slitting would cause at least one of the narrower webs to jam the rewinder. This required shutdown and cleaning out the jam. To avoid this recurring problem of dealing with "fish-eyes" in the web, the web was rewound as a log. So, to cut the log into the retail size lengths, a log saw was needed.
The first commercially successful log saw was the "Gilbertville" saw of Gage, U.S. Pat. No. 2,752,999. This saw operated intermittently--swinging downwardly and upwardly (in knife-like fashion) against the log which was indexed a product length (41/2" for bathroom tissue) while the saw disc was lifted upwardly way from the log path. Sharpening of the saw disc occurred also when the disc was upwardly out of the log path.
The next commercially successful log saw is seen in co-owned U.S. Pat. No. 3,213,731. This differed from the Gilbertville saw in having the saw disc move through an orbit--passing through the log or logs at the nadir of the orbit. Sharpening occurred when the disc was upwardly out of the path. However, as with the Gilbertville saws, the operation was still intermittent--only having the saw cut the log after it had been indexed the appropriate distance.
The next major development was in the mid-1970s with the use of the continuous motion (CM) saw as seen in co-owned U.S. Pat. No. 4,041,813. This differed in two major respects from the prior log saws. The log was advanced continuously and the saw was orbited continuously. This was achieved by having the orbit axis skewed relative to the path of the log travel and providing skew compensation--to dispose the blade at a right angle to the log. The angle of skew was of the order of a few degrees--sufficient to accommodate the space in between cuts. Until the instant invention, the '813 saw represented the state of the art. A saw of competitive manufacture is seen in U.S. Pat. No. 5,315,907 where the movement was reciprocal, not continuous.
It has always been necessary to maintain a razor-like sharpness on the cutting edge of the blades. To do this, the sharpening system of the '813 patent had to be mounted on the angled housings and had to follow the planetary motion. Because the sharpening stones or grinders used for sharpening were mounted out near the blade's edge, each blade/grinder assembly was difficult to balance, especially due to the changing position of the grinders as the blade diameter decreases. Since the system was generally out of balance, the planetary gear train had to deal with the constant imbalance torque and its cyclic nature, reversing once each revolution. The planetary motion also put the grinders into completely reversing cyclic loading causing component fatigue and grind quality problems as the production speed increased.
In particular, the design of the '813 cutter head was complex both in the drive and in the blade orienting planetary system. However, no better CM saw was available for twenty years and the drawbacks of cyclic loading due to planetary motion had to be tolerated.
The problem, therefore, was to produce this same type of blade action but without the use of planetary motion. For this, the invention provides a motion that allows for locating the grinders at a lesser orbit radius than the blade center and leaves them always toward the center of rotation, thereby eliminating the cyclic centrifugal forces. More particularly, the invention involves positioning the grinding stone or stones radially inward of the blade orbit.
This, by itself, was well known via co-owned U.S. Pat. No. 4,584,917 but this was not for a CM saw where the stone cyclic loadings were encountered. Rather, the '917 saw had an intermittent feed, viz., "indexing" which was characteristic of the pre-'813 saws.
Representative of the prior art for sharpening orbiting knife blades operating in slots in a wheel is in the field of yarn cutting--U.S. Pat. No. 3,218,898. The knife blades rotate slower than the turntable which carries them and the sharpening grindstones which sharpen the blades constantly throughout the operation. This is in the field of yarn cutting but it teaches the sharpening of the blades constantly throughout the cutting operation. This is because periodic sharpening was only satisfactory at low speeds. In '898, this was due to the impact velocity of the knife upon the stone being sufficient to make the stone rebound away from the knife and not allow the knife to be sharpened.
More significant is the fact that the '898 saw was not even capable of keeping a blade perpendicular to the product. Because it was cutting filaments, the cutting duration was an instant in time and squareness of cuts do not matter nor are measurable. Even further, the orbit in '898 was not skewed to the product and the reference even states that the orbit is normal to the filament. From a cutting aspect, this saw is more primitive than a Gilbertville saw. In contrast, the invention provides a grinding system which will provide periodic sharpening, as continuous sharpening is not wanted due to excessive blade heat, then blade warps, and ultimately the blades become out of round.
The invention is described in conjunction with illustrative embodiments in the accompanying drawing.